1. Field of the Invention
This invention relates a cold mirror for a head-up display (hereinafter abbreviated as “HUD”) apparatus in which change in color tone of an image depending on a position from which the image is viewed is reduced. This invention also relates to a HUD apparatus including the cold mirror.
2. Description of the Related Art
A HUD apparatus is, for example, mounted on a vehicle for use in displaying information such as infotainment information and/or ADAS (Advanced Driver Assistance Systems) in a field of view on the front side of a driver. Generally, such in-vehicle HUD apparatus has a configuration in which an image of, e.g., drive information luminously displayed on a display such as a liquid-crystal display is reflected by a mirror to project the image onto a windshield or a combiner in front of a driver. In such in-vehicle HUD apparatus as configured above, upon sun light reversely entering and being reflected by the mirror and then entering the display, the display deteriorates as a result of being heated. Therefore, use of a cold mirror for forming the mirror has conventionally been proposed (for example, Japanese Patent Laid-Open No. 2004-347633 described below). As a result of using a cold mirror for forming the mirror, even if sun light reversely enters, infrared components included in the sun light are absorbed by the cold mirror and the display is thus prevented from being heated.
A test conducted by the present inventor showed that a conventional HUD apparatus including a mirror formed of a cold mirror has a problem in that an image changes in color tone depending on a position from which the image is viewed. For example, if a white image is displayed on the entire display of the in-vehicle HUD apparatus, while the entire display looks white from a viewpoint of a driver, the display sometimes looks somewhat colored from a front passenger seat or a rear seat.
This invention provides a cold mirror for a HUD apparatus, the cold mirror solving the aforementioned problem of the related art and suppressing change in color tone of an image depending on a position from which the image is viewed. This invention also provides a HUD apparatus including the cold mirror.